In the treatment of waste paper, it is increasingly the practice to recycle the requisite water in order to keep water consumption as low as possible and to avoid environmental pollution.
However, particularly in flotation plants, a closed water cycle suffers the problem that soap, fatty acid or other chemicals cause a gradual reduction in the water hardness. The soap or other chemicals used during the flotation process must be converted into insoluble calcium soap by water hardening agents contained in normal water. If the water is not hard enough and there is too little calcium soap in the water, less printing ink is removed from the paper. The resulting paper is then dark and this makes it difficult to use the resulting waste paper in a paper machine. In particular, the fatty acid released from the soluble sodium soap during acidification can cause stickiness.
Previously, the problem was overcome by introducing calcium chloride into the closed water cycle. Calcium chloride can be easily dissolved in water and it forms calcium ions, which increases the water hardness appropriately. However, the calcium chloride in the water also produces chlorine ions, which then reacts with the sodium ions of the soap producing considerable quantities of sodium chloride. Slightly more sodium chloride is formed than the amount of calcium chloride that is introduced. This means that, for example, in a waste paper treatment plant which treats 100 tons of waste paper per day, approximately 300 kg sodium chloride could be formed daily. This quantity of sodium chloride often leads to extremely unpleasant corrosion phenomena throughout the entire plant.